This invention relates to antenna systems, and, more particularly, to the providing of an antenna adapted for operation in multiple bands.
It is common to use a single antenna array to provide a radiation pattern, or beam, which is steerable. For example, steerable beams are often produced by a planar or panel array of antenna elements each excited by a signal having a predetermined phase differential so as to produce a composite radiation pattern having a predefined shape and direction. In order to steer this composite beam, the phase differential between the antenna elements is adjusted to affect the composite radiation pattern.
A multiple beam antenna array may be created, utilizing a planar or panel array described above, for example, through the use of predetermined sets of phase differentials, where each set of phase differential defines a beam of the multiple beam antenna. For example, an array adapted to provide multiple selectable antenna beams, each of which is steered a different predetermined amount from the broadside, may be provided using a panel array and matrix type beam forming networks, such as a Butler or hybrid matrix.
When a planar array is excited uniformly (uniform aperture distribution) to produce a broadsided beam projection, the composite aperture distribution resembles a rectangular shape. When this shape is Fourier transformed in space, the resultant pattern is laden with high level side lobes relative to the main lobe. Moreover, as the beam steering increases, i.e., the beam is directed further away from the broadside, these side lobes grow to higher levels. For example, a linear array with its beam-peak at "THgr"0, can also have other peak values subject to the choice of element spacing xe2x80x9cdxe2x80x9d. This ambiguity is apparent, since the summation also has a peak whenever the exponent is some multiple of 2xcfx80. At frequency xe2x80x9cfxe2x80x9d and wavelength lambda, this condition is       2    ⁢          π      ⁡              (                  d          λ                )              ⁢          (                        sin          ⁢                      xe2x80x83                    ⁢                      Θ            scan                          -                  sin          ⁢                      xe2x80x83                    ⁢                      Θ            0                              )        =      2    ⁢    π    ⁢          xe2x80x83        ⁢    p  
for all integers p. Such peaks are called grating lobes and are shown from the above equation to occur at angles "THgr"p such that sin "THgr"p=sin "THgr"0=2xcfx80p. Accordingly, when the radiation pattern is steered too far relative to the element spacing a grating lobe will appear which can have a peak in its pattern nearly equal to the main lobe of the radiation pattern. The point at which this occurs is generally considered the maximum useful steering angle of the array.
Even when steering of the main beam is restricted to angles such that the grating lobe presents a peak appreciably less than that of the main lobe, the presence of the grating lobe acts to degrade the performance of the antenna system by making it responsive to signals in an undesired direction, potentially interfering with the desired signal. Specifically, as the main beam is steered off of the broadside of the array, the grating lobe will often be directed at an angle within the range of angles the antenna array is operable within. Accordingly, the presence of a stray communication beam having a substantial peak associated therewith and present within the area of operation of the antenna array will very often be a source of interference. Moreover, as the grating lobe is substantially coaxial with the axis of radiation of the antenna panel, it is generally not possible to avoid this interference with solutions such as tilting the array to point the grating lobe in a harmless direction.
Additionally, broadside excitation of a planar array yields maximum aperture projection. Accordingly, when such an antenna is made to come off the normal axis, i.e., steered away from the broadside position which is normal to the ground surface and centered to the surface itself, the projected aperture area decreases causing a scan loss. This scan loss further aggravates the problems associated with the grating lobes because not only is the aperture area of the steered beam decreased due to the effects of scan loss, but the unwanted grating lobes are simultaneously increased due to the effects of beam steering.
It is sometimes desirable to utilize a particular antenna aperture for communication of multiple services and/or frequency bands. For example, zoning restrictions and other concerns may limit communication service providers ability to deploy separate antenna systems for use with various communication services, such as standard cellular telephony services and personal communication services (PCS). Accordingly, it may be desirable to provide a single antenna system to service multiple such services.
However, it should be appreciated that each such service may utilize a substantially different frequency bands, e.g., the aforementioned standard cellular systems may operate at approximately 800 MHz whereas PCS systems may operate at approximately 1.8 GHz. Therefore, undesirable antenna attributes, such as the aforementioned grating lobes, may be experienced to differing degrees in association with each of the multiple services, making design and implementation of a single antenna aperture for use with multiple services challenging.
Accordingly, a need exists in the art for a system and method of providing antenna beams having a desired beam widths and azimuthal orientations without suffering from the presence of grating lobes when steered a desired amount off of the broadside.
Moreover, as multiple beam antenna arrays are useful in providing wireless communication networks, such as standard cellular services and/or personal communication services (PCS) networks (referred to hereinafter collectively as cellular networks), which are often simultaneously provided in a same service area, a need exists in the art for the systems and methods adapted to provide desired antenna beams substantially free of grating lobes to also be adapted for dual mode service.
These and other objects, features and technical advantages are achieved by an antenna array, such as a multiple beam antenna system including a beam forming matrix, wherein only the inner most beams of those possible from the array are utilized and the pertinent antenna element column or row spacing is adjusted to achieve the desired antenna beam shapes, i.e., beam widths, and sector pattern. The radiation pattern resulting from the use of such an antenna, whether relying on restricted beam switching of a multiple beam array or restricted scanning of an adaptive array, utilizing only the inner beams has the desired characteristic of avoiding the grating lobes associated with the outer most antenna beams, or other antenna beams steered substantially from the broad side, of an array.
An antenna array for providing desired communications may use four beams, i.e., a panel having four antenna columns provides four 30xc2x0 substantially non-overlapping antenna beams which when composited provide a 120xc2x0 sector. The beam forming matrix for such an array may be a 4xc3x974 Butler matrix, a matrix having inputs and outputs limited to powers of two (inputs/outputs=2, wherein n=2 for the 4xc3x974 matrix), providing the signals of four antenna beam interfaces in a phased progression at each of the four antenna columns. These beams may be referred to as, from left to right viewing the antenna array from the broadside, 2R, 1R, 1L, 2L, with the beams steered at the most acute angle off of the broadside, beams 2R and 2L, having substantial grating lobes associated therewith.
A preferred embodiment of the present invention utilizes an antenna capable of providing antenna beams steered further off of the broad side than those relied upon for providing communication. For example, a preferred embodiment utilizes a beam forming matrix having 2n+1 inputs for forming 2n antenna beams. Accordingly, in the above example where four (22) beams are desired, a beam forming matrix having eight (23) inputs and outputs is utilized. In order to provide the desired beams without the presence of grating lobes while still providing tolerable side lobe levels, and a desirable main beam, the antenna array fed by the beam forming matrix of this embodiment of the present invention has a number of antenna columns corresponding to the n+1 inputs. Therefore, the eight outputs of the beam forming matrix are each coupled to one of eight antenna columns of an antenna array and is thus capable of providing eight antenna beams (4R, 3R, 2R, 1R, 1L, 2R, 3R, and 4R).
According to the present invention, although the antenna array may be capable of forming a number of beams in excess of those desired, only the inner beams are used. For example, in the preferred embodiment described above only the 2R, 1R, 1L, and 2R beams are used out of an available combination of 4R, 3R, 2R, 1R, 1L, 2L, 3L, and 4L beams. These inner most beams typically have better radiation characteristics than the outer most beams and therefore do not present the grating lobes it is a purpose of the present invention to avoid.
However, it should be appreciated that the characteristics of the individual antenna beams of the above described array of the present invention will not substantially conform to those of the antenna array it is intended to replace. For example, rather than providing four approximately 30xc2x0 antenna beams which define a 1200 sector, the 2R, 1R, 1L, and 2R beams as of the 8xc3x978 beam forming matrix used according to the present invention may provide four approximately 150 antenna beams which define a 60xc2x0 sector because of the increased number of antenna columns energized in the phase progression.
Accordingly, the present invention, includes adjustment of the antenna column and/or row spacing to re-point the used beams in the desired direction although the phase progression utilized for a more narrow beam eight beam array are maintained. Moreover, as the inter column spacing is adjusted to re-point the beams at desired angles from the broadside, so too are the antenna beam widths adjusted to desired widths. Accordingly, the above described preferred embodiment antenna array having an 8xc3x978 beam forming matrix may be utilized to provide four substantially 30xc2x0 beams defining a 1200 sector.
The respacing of antenna elements according to the present invention results in the closing in the elemental spacing which has the desirable effect of reducing or even suppressing any grating lobes that may have been present in the original array configuration. It should be appreciated that the respacing of antenna elements, by closing in the elemental spacing, of the preferred embodiment may result in undesirable effects associated with the phenomena of mutual coupling. Accordingly, preferred embodiments of the invention use techniques to over come adverse effects of mutual coupling associated with antenna elements being placed in close proximity to one another.
For example, embodiments of the present invention employ the use of xe2x80x9cstaggerxe2x80x9d tuning. Additionally or alternatively, embodiments of the present invention employ the use of electrically grounded partitions, referred to herein as xe2x80x9cFaraday fencesxe2x80x9d. These two very different techniques may be used according to preferred embodiments of the present invention to over come the effects of mutual coupling between the radiating elements making up the antenna array which can distort individual element patterns that are components in the process of beam forming. For example, either or both of the above techniques can be used for mitigation of direct space coupling. Faraday fences may be used along row and/or column spacings of an array to provide isolation between adjacent elements while providing for the use of a uniform feed system, such as may be particularly desirable for a mass-produced antenna product by minimizing the need for different parts.
Further, the use of a Butler matrix as well as individual element, column, and/or row impedance matching can be used to minimize coupling associated with the feed network that interconnects elements in the array. Keeping the installation of the antenna away from blocking structure, such as an associated support tower, may be utilized in minimizing indirect coupling occurring by scattering from nearby objects.
Elemental spacing according to the present invention may be adjusted to affect the best possible compromise between independent modes, such as advanced mobile phone services (AMPS) and code division multiple access (CDMA) communication signals, that may be using the array simultaneously. Additionally or alternatively, embodiments of the present invention provide a first group of antenna elements, preferably having the above described reduced spacing, for use with a first communication service or frequency band, and a second group of antenna elements, also preferably having the above described reduced spacing and interspersed with the first group of antenna elements, for use with a second communication service or frequency band. Accordingly, the geometry of each such group of antenna elements may be tuned for the respective communication service or frequency band used therewith. This interspersed element dual band configuration provides an antenna system having a single antenna aperture for multiple communication services which may be substantially the same size as that of a single communication service antenna array.
Preferably, the antenna elements of each such group of interspersed antenna elements are disposed in a same plane. For example, the antenna elements of each such group may be disposed in a plane parallel to and a quarter of the low band (e.g., first frequency band) mid-frequency wavelength above a ground plane. However, the antenna elements of each antenna element groups are preferably disposed a quarter of their respective band mid-frequency wavelength above a ground plane. Accordingly, a preferred embodiment of the present invention provides adaptation of the antenna ground plane to present a ground plane surface, such as a raised fin corresponding to antenna elements of the second group of antenna elements, a quarter of the respective band mid-frequency wavelength behind each antenna element to thereby allow each antenna element to be disposed in the same elemental array plane while providing the desired ground plane relationship with respect to elements of each communication service or frequency.
Preferred embodiments of the interspersed element dual band antenna array include antenna elements in addition to those directly used in the desired improved beam forming. For example, the interspersing of antenna elements of the different groups of antenna elements may affect communication using one or the other antenna element groups, such as by resulting in a non-uniform radiating environment. Specifically, the antenna elements of one group of the antenna elements present somewhat parasitic radiating structures with respect to antenna elements of another group of antenna elements of the above embodiment. Accordingly, antenna elements of inner columns of a group of antenna elements may be presented an appreciably different radiating environment than antenna elements of outer columns of a group of antenna elements. Accordingly, a preferred embodiment array of the present invention provides additional antenna elements disposed to provide a quasi-uniform radiating environment as seen by the active antenna elements. According to a preferred embodiment of the invention, these additional elements may be utilized in various ways in addition to providing a uniform radiating environment, such as to provide antennae for use in an opposite link direction with respect to the aforementioned grouped antenna elements.
Although described above with respect to an antenna array utilizing a beam forming matrix having a number of inputs associated with multiple antenna beams, an alternative embodiment of the present invention utilizes an adaptive beam forming matrix in combination with the array having additional columns and respaced antenna elements in order to provide a steerable antenna beam which, when steered significantly off broadside, has little or no grating lobe associated therewith. Such an embodiment preferably relies upon a feed network dynamically providing a phase progression across the antenna columns rather than the fixed phase progression of the above mentioned Butler and hybrid beam forming matrixes. Accordingly, it should be appreciated that the phase progression provided by this adaptive feed network is consistent with that of the more narrow beams of the larger array, although utilized to provide a lesser number of improved beams according to the present invention.
A technical advantage of the present invention is to use a phased array antenna to provide multiple or steerable antenna beams with reduced or no grating lobes.
A further technical advantage of the present invention is to provide an antenna which is optimized for use in communicating multiple communication modes simultaneously.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.